The invention relates to a discharge lamp and an outer bulb for a discharge lamp.
Discharge lamps generate light in that an electrical discharge is induced in a closed discharge vessel, usually of small volume surrounded by a glass mantle. To this end the discharge vessel is filled with a gas which contains the light-emitting metal, sodium in particular. By the imposition of an electrical voltage, usually at two electrodes, a discharge is excited which emits a strong light.
The principle is known of surrounding the discharge vessels of such discharge lamps with a further casing of glass. For example, in the case of discharge lamps for automobile headlights the actual discharge vessel, often oval in shape, is frequently surrounded by an outer bulb, such as a cylindrical glass tube. The inner walls of the glass tube are frequently very narrow, with the result that the distance from the discharge vessel is in part very small (0.2 to 1 mm, for example). In part, it may even occur that the discharge vessel and the outer bulb come into contact with one another. The outer bulb is frequently permanently connected to retaining elements of the discharge vessel. Inside the outer bulb is a gas volume, which is usually filled with air.
The function of the casing (outer bulb) around the discharge vessel consists on the one hand of easing the thermal load on the discharge vessel. The discharge vessel reaches high temperatures during operation. Due to the formation of a zone of high temperature in the interior of the outer bulb, the temperature differentials in the area of the discharge vessel are not so great as would arise without the casing. Another function of the outer bulb is frequently the filtering out of UV radiation from the light of the discharge lamp. To this end, the principle is known of doping the glass material of the casing with various different materials in order to improve the UV absorption.
In U.S. Pat. No. 5,541,471 a discharge lamp is described which is suitable as lighting for an automobile, in which a casing surrounding the discharge vessel consists of a quartz glass which is doped with the elements cerium, titanium, europium, and aluminum, so that good permeability is provided for light from the visible spectrum, but ultraviolet light is absorbed. The casing in the form of a glass bulb in this case surrounds the discharge vessel.
EP-A-558270 likewise describes lamps with a casing of quartz glass. High-purity quartz sand is doped with TiO2 and CeO2 in order to achieve better absorption of UV light. The quartz sand used contains a number of different elements as impurities, among them the alkaline metals K and Li at 0.5 ppm and Na at 0.6 ppm.
The principle is also known of doping the glass of the casing in order to influence the processing properties. In EP-A-0601391 a doped quartz glass is described, and its use as a casing body for lamps. In addition to high-purity SiO2, the quartz glass contains in particular earth alkali oxides and boron oxide, in order to reduce the viscosity of the glass. Because of the tendency of alkali oxides to evaporate at high temperature, it is proposed that these be added only in small quantities, and preferably to do without them altogether.
During the operation of discharge lamps the problem has arisen that the properties of the lamp are not constant over its service life. As the duration of operation increases, so the amount of light discharged by the lamp diminishes. Color shifts are also to be observed.
EP-A-0583122 discloses a gas discharge lamp with an outer glass bulb and a discharge vessel. As a cause for the change of the properties of the lamp during its service life, in particular the shift in color, the diffusion of sodium from the atmosphere of the discharge vessel is cited. In order to counter this problem of the diffusion of ions, a coating of metal silicate is applied to the inner side of the glass discharge vessel.
U.S. Pat. No. 5,631,522 describes a gas discharge lamp with a discharge vessel. In order to avoid the diffusion of sodium from the discharge vessel, a special composition is proposed for the glass of the discharge vessel, namely high-purity quartz glass or synthetic silicon oxide doped with 20 to 1000 ppm yttrium or caesium in each case, preferably in combination with aluminum oxide. It is indicated that the presence of alkali metals in the glass of the discharge vessel favours the diffusion.
It is an object of the invention to describe a discharge lamp and an outside bulb for a discharge lamp, whereby the lamp can be manufactured easily and economically and will maintain the most stable characteristics possible during its service life.
The object is achieved by a discharge lamp as claimed in claim 1 and an outer bulb for a discharge lamp as claimed in claim 5. Dependent claims relate to advantageous embodiments of the invention.
Surprisingly, it has transpired that the change of the properties over a long period of operation, such as two thousand hours burning, can be decisively influenced by the suitable selection of the material of the casing surrounding the discharge vessel. This is particularly surprising because this casing, also referred to hereinafter as the outer bulb, does not have any direct contact with the actual discharge. It is therefore an advantage of the invention that no elaborate treatment is required at the actual discharge vessel, such as the application of blocking layers. The discharge vessel can be optimised in accordance with the usual requirements, without separate consideration needing to be given to the problem of the diffusion of Na ions. For preference, the casing is arranged at a distance from the discharge vessel, while a space remains between the discharge vessel and the casing. This space can be enclosed, and is filled preferably with air.
According to the invention, the glass material of the casing is doped with a small quantity of sodium. It is assumed that during operation the following reaction will take place: like the actual discharge vessel, and especially if the outer bulb is in close proximity, the casing will become very hot. The temperature in this case may rise to more than 650xc2x0 C. In this situation a diffusion of the Na takes place from the material of the casing, with the result that this is located in the space between the discharge vessel and the casing. This concentration counteracts the diffusion of sodium ions from the discharge vessel.
According to the invention, sodium should be present in a concentration of at least 10 ppm, by relation to the weight, in the glass material of the casing. An upper limit for the Na content is determined by the properties required for the glass, in particular its workability and temperature stability. For lamps in which the outer bulb heats up to more than 650xc2x0 C. (because of the high temperature of the discharge vessel and the small interval gap), an Na content of less than 2000 to 3000 ppm is preferred. A concentration of at least 30 ppm is particularly preferred.
According to a further embodiment of the invention, provision is made for the material of the casing to contain further alkali metals (except for sodium) but only in very small concentrations. The total of the alkali metals (except for Na) should be a maximum of 25 ppm, and preferably even less than 15 ppm. In particular, the concentration of potassium should be a maximum of 10 ppm, but for preference even less than 6 ppm.
The background to this is the role which the alkali metals, and potassium in particular, is presumed to play in the diffusion of the Na ions from the discharge vessel. These alkali metals may likewise evaporate at the high temperature of the casing during operation, and may be replaced by Na atoms on the surface of the inner bulb. The diffusion of Na ions is accordingly favoured.